Autosomal-dominant (ADPKD) is the most common life-threatening genetic disease. Tuberous Sclerosis Complex (TSC) is 10-times less common than ADPKD. Both diseases are characterized by renal cysts. No treatment is currently available to slow the onset or progression of either disease. We have recently found that polycystin-1 (PC1), the protein affected in ADPKD, interacts with tuberin, the protein affected in TSC, and regulates the activity of the kinase mTOR. mTOR plays an important role in cell growth and proliferation and is known to be inhibited by tuberin. We propose a model in which the C-terminal cytoplasmic tail of PC1 functions to induce the formation of a complex between tuberin and mTOR and other regulatory proteins. The function of this complex is to suppress mTOR activity in normal renal epithelial cells. Our results suggest that mTOR inhibition is regulated by apical fluid flow. We found that mTOR is inappropriately active in the kidneys of ADPKD patients and in mice with polycystic kidneys. Treatment of polycystic kidney mice with the mTOR inhibitor rapamycin results in the dramatic reduction of renal and cyst sizes and in the preservation of kidney function. This strongly suggests that aberrant mTOR activation in ADPKD is critical for renal cyst growth. We now propose to study in detail the mechanism of the suppression of mTOR activity by PC1. In Aim 1, we will investigate the interaction between PC1 and mTOR and identify additional regulatory factors and the subcellular localization of the complex. We will test whether this complex is disrupted in renal cysts. In Aim 2, we will test how fluid flow regulates mTOR activity. In Aim 3, using samples from ADPKD kidneys and a mouse model with a conditionally inactivated PKD1 gene we will characterize which of the known downstream effectors and upstream regulators of mTOR are affected in the disease. In Aim 4, in preparation for possible future clinical trials, we will investigate the effects of rapamycin on the PKD1 mouse model. We will assess dosage and treatment regimens and investigate the effects on proliferation and apoptosis of cystic epithelial cells. These investigations will be important to guide possible future clinical trials. Rapamycin is already a clinically approved drug, used for long-term treatment for immunosuppression in renal transplant patients. Based on our data, we are cautiously optimistic that rapamycin has promise to become the first available treatment to slow the disease progression in ADPKD. Furthermore, results from this research will enhance our understanding of the mechanisms underlying the renal involvement in TSC.